Do sloths dump in the trees?

The answer is no. This delicate issue arose in our video and subsequent discussion of sloths: the beast has the odd habit of defecating on the ground, at the base of the tree it inhabits. That means a looooong, slow climb down from the branches. And, of course, you’ve seen from that video how awkward sloths are on the ground, so the descent exposes it to all sorts of predatory dangers. Fortunately, sloths digest their food (leaves) as slowly as they move, so they have to make the round-trip only about once a week.

Here’s an Attenborough video showing sloths at the loo, with His Holiness noting that the behavior is a complete mystery:

Given the difficulty and time involved in the trip, and the helplessness of sloths on the ground, why do they do it? By “why”, of course, I mean what were the advantages of any genes that produced this behavior? I’m assuming here that this behavior is genetically based rather than simply learned, which seems a reasonable supposition. I can think of four evolutionary explanations:

It reduces predation from above. Eagles and other aerial predators are said to detect the sound or appearance of droppings, using them to cue in on the sloth as prey. By hiding its toilet at the base of the tree, it makes itself less liable to aerial attack. I don’t find this very plausible given that aerial predators hunt visually, and I don’t see how they could detect a sloth more readily when it’s defecating from a branch. Indeed, it seems like it would be more likely to detect a sloth climbing down the tree.

It reduces predation from below. This seems more likely to me than alternative #1. Terrestrial predators like jaguars can hear droppings striking the forest floor, and seem likely to be able to associate them with prey above. Cats, for example, are known to climb trees to take sloths (if you’re not squeamish you can see a video of that here.) On the other hand, couldn’t a cat smell sloth droppings and use them as a way to hunt?

It fertilizes the sloth’s tree. I’ve heard this bandied about, but it doesn’t seem credible. Sloths hang out in big trees, and I’m not sure that a sloth with genes to move to the base and deposit fertilizer would really gain a reproductive advantage. That presumes that that such fertilizing would make such a substantial difference in the tree’s output of leaves that the sloth would wind up better fed and have more offspring. In addition, I’m not sure (though perhaps a reader can tell me) whether sloths remain in the same tree for years, as is required by this hypothesis.

It’s a way to attract mates. Creating your own personal dung pile may be the equivalent of expelling pheromones, alerting sloths of the opposite sex that you’re up above. I know nothing about sloth mating, but given their lassitude and site-fidelity, surely locating a mate—the most important behavior in outcrossing species—is subject to strong selection. How do you find another sloth two trees over? By sniffing the base of the tree.

The last explanation seems the best to me, though of course the reproductive advantage of sloth toiletry could have involved more than one of these factors. And in principle these theories are testable. We could see, for example, how sloths manage to find each other at mating time. We could also do mock-defecation studies from branches, using model sloths, to see if the noise attracts predators.

How about the ancestral position of the anus in the ground dwelling sloth made it simple to shit without getting it all over yourself. As the sloth took on an aboreal lifestyle it was easier to be hardwired to poop ‘standing up’ necessitating the climb down the tree than to move the location of the anus to a spot that prevented the sloth from shitting on itself (which presumably would increases disease and be selected against).

I think you’re right, they do it that way because they’ve always done it that way. Evolution happens by accident so maybe the sloth hasn’t had that accident yet. Could be that its ancestor was ground dwelling and did its dumping there. It doesn’t have to be adaptive.
Why do kiwis lay such big eggs? Fossil evidence suggests that they have descended from much larger birds. The smaller adult size seems adaptive but the genes for smaller eggs hasn’t happened yet.

Yes, but the behavior itself seems palpably MALADAPTIVE if there’s no compensating benefit. You might be right, but if there’s a cost to climbing down and then up, you’d expect that variants who would simply let fly from the branches would have an advantage (less predation and the like).

The behavior may be maladaptive, but so is being covered in your on feces. It may be worse if you’re covered in hair. Maybe sloths don’t have the genetic space to move their anus like whales/dolphins did with the blowhole.

This hypothesis (Lorax’s, with your amplification) was the first I thought of as well…

It occurs to me, though, that unless its anus were in a very peculiar location, a sloth should probably be able to orient its body while in the tree–say, by clinging to a relatively vertical branch–so that feces would fall away.

My favorite quote about slow-moving sloths is from Bertrand Russell in his excellent 1943 exposition of “intellectual rubbish” (including, of course, Biblical literalism):

“We may enjoy the perplexity of the South American Jesuit who wondered how the sloth could have traveled, since the Flood, all the way from Mount Ararat to Peru – a journey which its extreme tardiness of locomotion rendered almost incredible. A wise man will enjoy the goods of which there is a plentiful supply, and of intellectual rubbish he will find an abundant diet, in our own age as in every other.” An Outline of Intellectual Rubbish (1943)

I think it’s regional, rather than national. I’m considered pretty well-spoken by my colleagues, being a southerner who works in the Midlands, but I still pronounce those “eh-volution” and “sloh-th” with short vowels.

– use mating calls so I doubt they descend in hopes of a brief encounter. I would suppose that the home tree has a selection of potential partners. I wonder if young males are expected to find new trees or do they stay in a ‘family’ tree ?

– they urinate upside down ‘in situ’, but wait for it to rain (to cover the noise ? to wash the urine from their fur ?). The fur runs in the opposite direction to the norm to shed fluids while the animal is inverted

Well, if they bury their waste, we could suppose there’s a benefit to burying it. One candidate is that buried scat is much less of an indicator to predators that there’s a sloth nearby, for instance, up this tree. If they didn’t climb down to do it, it wouldn’t get buried unless they then climbed down just to do it – possibly right through a messy trail of it down the tree.

You beat me to it. I found this on one (not especially scientific-looking) website:

Sloths are not known as particularly social creatures, but they do spend enough time with the opposite sex to reproduce. One of the studies underway at Aviarios del Caribe when I was there involved the mating habits of three-toed sloths. We saw a video in which a female sloth named Buttercup let out a blood-curdling mating call that sounded like a woman shrieking. Immediately, males from as far away as 700 meters began rushing toward the sound. By “rushing,” I mean crawling at the breakneck speed of about 200 meters per day. But for a sloth, this single-minded, deliberate movement—on the ground, no less, in plain view of predators—is definitely rushing. Ah, the things we do for love.

As to several in a tree–the few I saw in Costa Rica were one per tree. They were not in terribly large timber, though–more Cecropia-sized trees..

I like the idea of exploring what ancestral/related extinct sloths did — It’s still just a story/hypothesis of course, but it’s a nonadaptive one which is at least different. Maybe there’s some weird neurological switch that requires having their bottom on the ground in order to do it, and for some reason evolution hasn’t gotten rid of it. That seems pretty unlikely to me, but it’s about as good as some of those other suggestions.

If I were NSF though I would vote for doing some serious long-term observation of sloths, is it *really* true that all sloths everywhere do only this? Even in zoos etc.? Anecdote can be a poor guide to reality…

If there were some comparative data, especially on fossil sloths, there might be something worth doing. E.g. I have a vague memory that the giant ground sloths tended to leave, ahem, deposits in caves. I’ve got a paleontology friend who is a slothologist I’ll ask…

Presumably, though, one could tell from location. In practice zookeepers do tend to develop a good sense of where the animals in their care like to crap, on account of how they’re supposed to find said crap and take it away. The nocturnal house “trees” were gunnite, so…

Must stop before I remember the scent of the nocturnal house too vividly. Nocturnal animals tend to be…pungent.

Perhaps the sloths (who carry many insects in their fur) do not want to be hosts to dung-eating insects? By leaving their dung as far away as they can safely deposit it the sloths misdirect the attentions of dung-eating insects (who may lay eggs which hatch into flesh eating grubs?)

If someone has carried out a census of the ‘on-sloth’ insect species this hypothesis could be tested…

My understanding is that much in evolution is neither adaptive or maladaptive, and is in fact neutral.

The idea here is that the sloth is more at risk on the ground than in the tree – so that a sloth that remains in the tree will have greater chances of reproductive success than other sloths.

The question that I’d like answered first is: How much greater?

Suppose that due to developmental reasons, defecating in the ‘cling to branch’ position might happen to be very uncomfortable. (I’m imagining doing this myself, and it doesn’t sound like a fun time).

My understanding is that developmental alterations are some of the most difficult changes for evolution to bring about.

Which in turn implies to me that the selection pressure would have to be very, very strong in order to shift that kind of developmental issue in favor of comfort when taking a crap from a tree.

Now – that’s all hand-waving, so I could easily be wrong.

But I wanted to illustrate the point that we need to know first how much of a selection pressure due to effort and predation would be placed on a sloth in this situation before we get too excited about demanding a rigorous explanation for it.

Then again – I have no expertise, so perhaps I’m greatly underestimating the risk to the sloth.

But if the sloth has control of where and when they come down… I dunno. It just doesn’t seem like it’s necessarily that big a deal. Some evidence to the contrary would be nice.

I once had the pleasure of recording a long interview with DA (on a portable Uher, I think it was) in Solomons Islands, when they were still The British Solomon Islands Protectorate.

At the end, to my horror, I found that the reels hadn’t turned. DA didn’t blink, didn’t call me out for an idiot for not making a test recording, but gave the interview again, almost word for word. A true professional (with so much experience the same thing has almost certainly happened to him at least once).

It would have been about what he was doing in the Solomons, which in 1974-6 would have been something to do with “Life on Earth”. I hope I also got him to give a defence of evolution; the Solomons are riddled with Creationism. Solomon Islanders are taught that evolution is wrong, but not what it is, and I was asked “What does evolution say about the Second Coming?”

Yeah– anti-constipation position. Lack of exercise can lead to constipation– so lack of exercise coupled with an upside-down posture… ‘a gravity-assist’ + exercise in the form of climbing down. That may well be the best idea yet!

Ok, Professor– how are we doing? Any better than the Ev. Psychologists?

I thought they mostly moved from tree to tree while still up– and that that was the reason we saw one trying to cross the road in the vid Jerry posted before. Human road=cut in trees, which the sloths can’t cross above and so must needs come down and cross to a new tree via the ground. Or?

An interesting aside here might be the semi-recent story of sloths being found (and taken out, but climbing back into) latrines in national parks. If it’s half-blind, half-deaf, maybe it’s also half-anosmiac and need a dung-heap in order to arouse the slightest chance of sexy-time?

A Bradypus, or Sloth, am I,
I live a life of ease
Contented not to do or die,
But idle as I please.
I have three toes on either foot,
Or half a doz. on both;
With leaves and fruits, and shoots to eat,
How sweet to be a Sloth.

The world is such a cheerful place
When viewed from upside-down;
It makes a rise of every fall,
A smile of every frown;
I watch the fleeting flutter-by
Of butterfly or moth [“mohth”]
And think of all the things I’d try
If I were not a Sloth:

[patter]
I could climb the very highest Himalayas,
Be among the greatest ever tennis players,
Win at chess or marry a Princess or
Study hard and be an eminent professor.
I could be a millionaire, play the clarinet,
Travel everywhere,
Learn to cook, catch a crook,
Win a war then write a book about it.
I could paint a Mona Lisa,
I could be another Caesar,
Compose an oratorio that was sublime!
The door’s not shut on my genius, but
I just don’t have the time!

For days and days among the trees
I sleep and dream and doze,
Just gently swaying in the breeze
Suspended by my toes.
While eager beavers overhead
Rush through the undergrowth,
I watch the clouds beneath my feet;
How sweet

Is there variation in this behavior among the existing species of sloths? There appear to be c. 6 species in two none-too-closely related groups (2-toed & 3-toed) and if they all show this behavior, then that implies it’s ancient, or strongly selected for in some way. (It seems the 2 groups separated 30 million years ago, or so).

If some climb to the ground and others don’t, there may be clues in that variation. Why has it evolved in some lines, but not others? What’s different about the environment? Or, why retain such an odd behavior so long in multiple lineages? Why hasn’t it broken down somewhere? Wouldn’t it be be simpler to flip around for a few minutes than to climb all the way down a tall tree?

Also — is there another arboreal mammal that bothers climbing down the tree to defecate? If not, doesn’t that undermine some of the arguments for why sloths do?

I’m sure I remember that sloths cross from one tree to another via the ground. It’s not easy to move through the trees via the canopy and I’d be surprised if they could manage it, which would resurrect the territory marking hypothesis. If a sloth spends most of its time in one tree and wishes to avoid an argument, it would make sense to alert other sloths to its location so they can keep going and find an empty one. They don’t look like the sort of animal which is interested in a fight so territory marking would save a lot of trouble all round.

The trees within its habitat must be close enough together for the sloth to proceed around the forest canopy when needed. The crowns of the trees in tropical forests are often thick with interlocking lianas and other vegetation and provide strong footholds for sloth travel, sleeping, mating and eating.

I’d been thinking along the same lines, though regarding dispersion on the ground. Perhaps part of being inconspicuous involves being solitary–i.e., that one sloth in a tree is less easily noticed than several sloths in a tree. Then a sloth on the ground seeking a new tree would benefit from knowing which ones were already occupied.

I was able to spend a couple of hours watching sloths feeding in Panama last year. They certainly can move from tree to tree in the canopy – I saw them do it twice. Whether it is the normal method of getting across or an occasional method I don’t know, but it is not rare (unless I was amazingly lucky).

One thing occurs to me. Sloths digest leaves the same way every other mammal digests plants, via bacterial action. I’m wondering if there’s an inoculum issue here. If a young sloth were to need to inoculated with the proper bacteria or an old sloth needed to be re-inoculated with bacteria, a good place to find said material would be in an old dung heap.

Gut temperature of the sloth is quite variable. They barely thermoregulate. This indicates their gut bacteria are not as dependent on internal temp as, say, a cow or a horse. Consequently, the period that an inoculum at the base of the tree would be viable might be quite long.

There could even be secondary paths. For example, the moth that lays its eggs in the feces and then comes back up to live on the sloth could bring an inoculum with her. The soil might contain trace elements required by the gut microbes. To go preposterously far with this, what we might be seeing is the sloths reaction to a microbial life cycle.

Oh, surely someone out there has sequenced the gut bacteria of the sloth!

Perhaps the poster was going by the fact that all the reputable looking sloth sites state that sloths in general don’t thermoregulate precisely and have a variable body temperature. (Google is our friend…)

Limited to equatorial habitats of constant high temperature climates and almost daily rainfall, Bradypus variegatus’s low and unstable body temperature varies depending on the temperature of its surroundings (Prosser and Brown 1961). With a body temperature as low as 34ºC in Bradypus (Goffart 1971) and commonly varying 10ºC during the day (Irving et al., 1942; McNab, 1978), continued exposure to the sun may result in death (Britton and Atkinson, 1938). On a cloudy day Bradypus variegatus’s body temperature does not exceed 5º above the air temperature (McNab 1978; Montgomery and Sunquist 1978). Because its body temperature is lower then most mammals studied, it barely keeps warm even in the tropics (Goffart 1971). One reason why Bradypus variegatus is not able to control its body temperature and is restricted to tropical climates is its reduced skeletal muscles which only contributes 25% -30 % of its overall weight (Goffart 1971). This small muscle mass effects its metabolism as Bradypus variegatus turns food into energy at half the rate of other species its size (Cavendish 1997). Their nerves and muscles function slower than most mammals as well (Goffart 1971).

Thank you. The expression “gut temperature” made me think they would have to have some kind of thermometer deep in the intestine (one of those radio capsule thingies,or a very long probe) but I guess blood temperature is a marker, depending on how hot the composting leaves get….

I am with the opinions above that it is because that is what theancestral ground dwelling sloth did, albeit that must have been a long time ago. Note that two-ted sloths are related to some recently extinct ground dwelling sloths.

Just because they defecate once a week, doesn’t mean that is how long it takes food to pass through the gut. Does anyone know how long? (Has anyone put a fluorescent dye on their leaves?) And how long is their small intestine?

One of the sites that mentioned the variable temperature of sloths also mentioned it took about a week to digest food.

I wonder if the bacterial processing of leaves by sloths is inefficient due to this temperature problem– ruminants, etc., keep a fairly constant temperature. I’m guessing sloths are hind gut fermenters like horses rather than foregut fermenters like cattle. There are many examples of hindgut fermenters.

If they had a limitation in their gut efficiency there might be a selective advantage to a slow metabolism. I’m also guessing the capability of gathering foodstuffs is less in trees than on the ground. Leaves are less abundant than grass.

I am doing the study and I can shed some light on this matter! To cut a long project short, I have been monitoring sloth behaviour,reproductive activity, thermoregulation and metabolic activity for the last 12 months and can tell you that the defecating at the base of the tree is almost certainly linked to finding a mate.

You wont find any literature on this at the moment but I can tell you female sloths have a very regular 28 day reproductive cycle, during which, they are in heat for 10 days. During these 10 days,they actually let out regular high pitched screams that attract males from up to 700m away, and they descend to the same spot at the base of the same tree daily to go poo/pee (even if it is just the tiniest amount!)

It also takes them an average of 31 days for food to pass from ingestion to excretion … amazing! And yes, their body temperature CAN fluctuate up 8 degrees over the course of a day. And the respiratory rate can range between 6 breathes per min to 120 !

Oh and the stuff about them being half deaf and half blind… completely false and I have no idea who spouted that one originally!! Very interesting animals :)

It may be another energy saving adaptation. It’s possible by being on the ground and being in the vertical position the sloth doesn’t have to put so much effort into the defecation process. If the sloth was defecating while holding onto the tree, then the forced contraction of the anus would mean having to hold on to the tree tighter, but given the lack of strength of a sloth, this would be counter productive.

To confirm this someone would need to examine how much effort the sloth is putting in during defecation.

It’s hard to believe the energy required by holding on more tightly would be balanced by the long climb to the ground and back again. Sloths spend a lot of time in the same tree.

I can see two possible reasons for the behavior. 1) The sloth is marking his territory or 2) the sloth is, in effect, encouraging the variety of tree it likes best by fertilizing it. Or both.

The latter idea is based on some eco-evolution research I read recently involving cottonwood trees out west. Turns out the comparative success of different varieties of the foundation species of cottonwood have marked knock on effects in the selection process of species dependent on that foundation species.

It doesn’t seem to far a stretch that a behavior in a grazing species have an effect on what it’s preferentially grazing. Be interesting to look at genetic expressions on dependent species like koalas and eucalyptus trees.

[…] Coyne appears to have embarked on a fascinating excursion into sloth ethology. At the end of the second installment, he invites readers to propose “evolutionary psychology” explanations for one specific […]